Conventionally, as general-purpose memories used in information processors such as computers or communications devices, volatile memories such as DRAMs (Dynamic Random Access Memories) or SRAMs (Static RAMs) are used. It is necessary to constantly supply a current to the volatile memories for maintaining memory and to perform refresh. When power is off, the volatile memories lose all information, so it is necessary to provide non-volatile memories for storing information in addition to the volatile memories, and as the non-volatile memories, flash EEPROMs, magnetic hard disk drives and the like are used.
As information processing becomes faster, an increase in the access speed of the non-volatile memories is an important issue. Moreover, the development of information devices for so-called ubiquitous computing, which means computing everywhere at anytime, has been rapidly advanced according to the rapid spread of mobile information devices and the enhancement of the performance of the mobile information devices. The development of non-volatile memories for high speed processing has been strongly desired as key devices which will be the center of the development of such information devices.
As an effective technique for increasing the speed of the non-volatile memories, a magnetic random access memory (hereinafter referred to as MRAM) in which magnetic memory devices each storing information by a magnetization direction along the easy magnetization axis of a ferromagnetic layer are arranged in a matrix form is known. In the MRAM, information is stored through the use of combinations of magnetization directions in two ferromagnets. On the other hand, stored information is read out through detecting a change in resistance (that is, a change in current or voltage) which occurs in the case where the magnetization direction is parallel to a reference direction and the case where the magnetization direction is antiparallel to the reference direction. As the MRAM operates on such a principle, in order to stably write or read information, it is important for MRAM to have as large a MR ratio as possible.
A currently practical MRAM uses a giant magneto-resistive (GMR) effect. The GMR effect is a phenomenon that in the case where two magnetic layers are disposed so that the directions of the easy magnetization axes of the magnetic layers are parallel to each other, when the magnetization direction of each magnetic layer is parallel to the easy magnetization axis, the resistance is minimum, and when the magnetization direction of each magnetic layer is antiparallel to the easy magnetization axis, the resistance is maximum. As an MRAM using a GMR device capable of obtaining such a GMR effect (hereinafter referred to GMR-MRAM), for example, a technique disclosed in the U.S. Pat. No. 5,343,422 is known.
The GMR-MRAM includes a coercivity difference type (Pseudo spin valve type) and an exchange bias type (spin valve type). In the coercivity difference type MRAM, a GMR device includes two ferromagnetic layers and a non-magnetic layer sandwiched between the ferromagnetic layers, and information is written and read out through the use of a difference in coercivity between the ferromagnetic layers. In this case, when the GMR device has, for example, the structure of “a nickel-iron alloy (NiFe)/copper (Cu)/cobalt (Co)”, the MR ratio of the GMR device is as small as approximately 6 to 8%. On the other hand, in the exchange bias type MRAM, a GMR device includes a fixed layer of which the magnetization direction is fixed by antiferromagnetic coupling to an antiferromagnetic layer, a free layer of which the magnetization direction is changed by an external magnetic field, and a non-magnetic layer sandwiched between the fixed layer and the free layer, and information is written and read out through the use of a difference between the magnetization directions of the fixed layer and the free layer. For example, the MR ratio in the case where the GMR device has the structure of “platinum-manganese (PtMn)/cobalt-iron (CoFe)/copper (Cu)/CoFe” is approximately 10%, which is larger than that in the coercivity difference type MRAM. However, the MR ratio is not sufficient to achieve further improvement in storage speed or access speed.
In order to overcome these issues, an MRAM including a TMR device which uses a tunneling magneto-resistive (TMR) effect (hereinafter referred to TMR-MRAM) has been proposed. The TMR effect is a phenomenon that a tunnel current flowing through an extremely thin insulating layer (a tunnel barrier layer) sandwiched between two ferromagnetic layers is changed according to a relative angle between the magnetization directions of the ferromagnetic layers. When the magnetization directions of the two ferromagnetic layers are parallel to each other, the resistance is minimum, and when they are antiparallel to each other, the resistance is maximum. In the TMR-MRAM, when the TMR device has, for example, the structure of “CoFe/aluminum oxide/CoFe”, the MR ratio is as high as approximately 40%, and the resistance is large, so the TMR device easily matches a semiconductor device such as a MOSFET. Therefore, compared to the GMR-MRAM, higher output can be easily obtained, and the improvement in storage capacity or access speed is expected. In the TMR-MRAM, a method of storing information through changing the magnetization direction of a magnetic film of the TMR device to a predetermined direction by a current magnetic field generated through passing a current through a lead is known. As a method of reading stored information, a method of detecting a change in resistance of the TMR device through passing a current in a direction perpendicular to a tunnel barrier layer is known. As techniques regarding the TMR-MRAM, techniques disclosed in the U.S. Pat. No. 5,629,922, Japanese Unexamined Patent Application Publication No. Hei 9-91949 and the like are known.
As described above, the MRAM using the TMR effect can achieve higher output than the MRAM using the GMR effect. However, even in the MRAM using the above-described TMR device having an MR ratio of approximately 40%, the output voltage is approximately a few tens of mV, so it is not sufficient to achieve a magnetic memory device with a higher density.
FIG. 40 shows a plan view describing the structure of a conventional magnetic memory device using the TMR effect, and FIG. 41 shows a sectional view of a main part of the conventional magnetic memory device corresponding to FIG. 40. A read word line 112 and a write word line 106, and a write bit line 105 are orthogonal to each other, and a TMR device 120 including a first magnetic layer 102, a tunnel barrier layer 103 and a second magnetic layer 104 is disposed between the read word line 112 and the write word line 106, and the write bit line 105 at an intersection of them. In such a MRAM in which the write bit line 105 and the write word line 106 are orthogonal to each other, the magnetization direction of the second magnetic layer 104 which is a free layer cannot be sufficiently aligned, so it is difficult to perform sufficiently stable writing.
In the MRAM using the TMR effect, the magnetization direction of the magnetic film is changed by an induced magnetic field by a current flowing through leads orthogonal to each other, that is, a current magnetic field, thereby information is stored in each storage cell. However, the current magnetic field is an open magnetic field (a magnetic field which is not magnetically trapped in a specific region), so the efficiency is low, and an adverse influence on adjacent storage cells is concerned. Moreover, in the case where the density of the magnetic memory device is further increased through further increasing the packing density of storage cells, it is necessary to reduce the size of the TMR device; however, the following issue is concerned. It is considered that when the aspect ratio (thickness/width in a in-plane direction of a laminate) of each magnetic layer in the TMR device increases, a demagnetization direction increases, and the magnitude of a magnetic field for changing the magnetization direction of the free layer increases, thereby a large write current is necessary.